The microstructure, mechanical properties, and high-temperature oxidation resistance of AlN/SiO 2 nanomultilayer coatings with various SiO 2 layer thicknesses were investigated using X-ray diffractometry, high-resolution transmission electron microscopy, scanning electron microscopy, and nanoindentation. The results revealed that SiO 2 formed wurtzite-typed hexagonal pseudo-crystal structures and grew epitaxially with AlN when its thickness was less than 0.6 nm. Meanwhile, the multilayer coatings yielded superhardness effect with a maximum hardness of 29.0 GPa. A minute increase in SiO 2 thickness from 0.6 to 0.9 nm would decrease the hardness of the nanomultilayer coatings due to the formation of amorphous SiO 2 and destruction of epitaxial structure. The high hardness of superhard coatings was sustained after high-temperature annealing treatment of up to 800°C. However, a further increase in annealing temperature to 900°C caused severe oxidation of AlN and thus degradation of coating's hardness.